The present invention relates to a clad steel in which an extremely low carbon steel used as a base metal is bonded together with an austenite stainless steel, nickel or nickel alloy as a cladding metal by a rolling or an explosive process, and more particularly to a clad steel having superior properties in which Ti and/or Zr are added to the base metal in order to fix carbon contained therein, or an extremely low carbon steel containing Ti and/or Zr is interposed between a base metal and a cladding metal in order to suppress the transfer of C by diffusion from the base metal to the cladding metal and in which the corrosion resistivity and the workability of the cladding metal are reserved, deterioration of the bonding force is prevented and the effective thickness of the cladding metal is maintained.
Although clad steel is required to make a cladding metal to preserve its original properties in order to exhibit necessary functions as products, when
(a) its bonding between the base steel and the cladding metal is carried out by pressure through a hot plastic deformation;
(b) it is hot worked after bonding: or
(c) it is subjected to a solid solution heat treatment in the form of a bonded metal; etc., in conventional clad steel any carbon contained in its base steel is transferred by diffusion or carburizing into the cladding metal so that the quality of the clad steel deteriorates. That is, according to the diffusion of carbon into the cladding metal, the decrease in its original properties such as a decrease in corrosion resistivity, deterioration in formability due to a decrease in malleability, etc. results.
Further, the diffusion of carbon into the cladding metal also acts to substantially decrease its effective thickness which must possesses its original properties. Therefore, a clad steel which has a thin layer of a cladding metal or a low cladding ratio can become a matter of great concern unless the phenomena of the diffusion of carbon solved.
In order to solve this phenomena hitherto various attempts have been proposed. For instance, a publication entitled "A Hand of Metal Surface Treatment Engineering", issued on Feb. 20, 1972 by the Daily Industrial News Papers, Page 1,097, describes a technique in which nickel or iron plating is interposed between a base steel and a cladding one.
Recently, Japanese Patent Publication No. 31544/1975 discloses a process in which in order to retard or prevent the carbon diffusion of steel comprising 5 to 40% of Cr, 16 to 50% of Ni, etc. an interlayer, i.e. barrier layer, is interposed between the base steel and the cladding one.
Japan Patent Laid-Open Patent Publication No. 2654/1976 also discloses a method for prevention of carbon diffusion by the use of a base steel which contains a smaller amount of C than that contained in the cladding metal.
U.S. Pat. No. 3,121,949 discloses a method in which in manufacturing a cladding steel from Ti and a mild steel by hot rolling, Cr, Mo, Ti, Nb, V and Zr are added to the mild steel to convert C in it to carbides so that a bonding having a ductility between Ti and the mild steel is realized.
As a most recent technique U.S. Pat. No. 3,956,809 discloses a method for prevention of carburization in which as a base metal a low carbon steel added with Ti and Nb in an amount of 5 to 15 times its carbon content is used.
Although various new processes have been developed as indicated above, it is true that the effects of a conventional electro-plating process can be recognized to some extent. In order to achieve good results by the electroplating process, however, it is necessary to increase the thickness of the electro-plating layer, which results in not only a problem in the strength of bonding, but also makes practical use difficult due to higher costs.
On the other hand, the process disclosed in Japan Patent Publication No. 31544/1975 as above described uses an alloy containing large amounts of Cr and Ni, so this process also makes the cost higher and it can be expected to obtain only an effect substantially similar to the electoplating process.
Also the process disclosed in Japanese Laid-Open Patent Publication No. 2654/1976, in reality, unless the amount of C in the base steel is small (e.g. 0.003% at the maximum) diffusion of carbon from the base steel to the cladding metal cannot be avoided due to the existence of a difference in affinity between Fe-C and Cr-C.
Although the method disclosed in U.S. Pat. No. 3,121,949 was tested, it was confirmed that when any metal other than Ti was used as a cladding metal effectiveness was exhibited, but when Ti was used as a cladding metal the effect was scarcely recognizable.
U.S. Pat. No. 3,956,809 provides a quite effective method. No substantial carburization is observed microscopically when the Ti/C ratio becomes larger than 8, although chemical analysis shows that the prevention of carburization is not perfect. When the cladding ratio is small, the carburization reaches as far as the surface of the stainless steel so that the corrosive resistivity and workability deteriorate. Of course, if the Ti/C ratio is less than 8 the prevention of carburization is imperfect. Further, owing to the addition of carbon fixing elements to the base metal, the yield strength of the base metal is decreased and its grain size is made larger than 3 so that the deep drawing characteristics of the clad steel should be improved. However, as shown in the examples, the grain size is practically 4 to 6, and such a grain size cannot be said to be perfect for deep drawing. Further, when the carbon content of the base metal is relatively large (in the examples the minimum value is 0.04%), the sum of the C-content and the carbon fixing elements make deep drawing impossible. In order to remedy this, strict caution must be exercised in the heat treatment after rolling. That is, since the sum of the C-content and the carbon fixing elements in the base metal is large, rapid cooling such as water cooling after the heating is not good for obtaining deep drawing characteristics. Mild cooling such as cooling in air or a furnace is required. However, for stainless clad steel, unless it is cooled as quickly as possible its corrosion resistivity and workability deteriorate, and since the carbon in the mild steel side has a tendency to carburize more into the stainless steel side during cooling after the heat treatment (mainly within the range of 600.degree. to 900.degree. C.), the method of U.S. Pat. No. 3,956,809 causes a problem in practice since it allows only mild cooling.
Although it is not so in the case of clad steels, the stabilization of carbon in stainless steels has been practiced already. For example, in the case of AISI 321, AISI 347 and SUS 430F, in order to stabilize the amount of carbon contained in them, Ti, Nb, Ta, and Zr are added to them, respectively.
Additionally, Japan Patent Publication No. 15004/1971 discloses a process for a melt-welding of steels of different kinds in which, in the case of welding the steels each containing a different amount of Cr, a welding portion which has a composition similar to that of the steel containing a smaller amount of Cr and which further contains Ti or Ti plus Nb in an amount up to 3 to 4 times the amount of carbon contained in the composite welding portion is formed between both steels, thereby the transfer of carbon due to diffusion in the boundary portion is prevented.